An electrical potential difference is measured between mother and fetus in most species. It is typically large enough to be a major force influencing water balance and the distribution of ions. Two theories have been advanced to account for the potential, neither of which is entirely satisfactory. Rather than postulate multiple ion exchange pumps, we propose a third alternative wherein an electric voltage is generated when H+ ions, products of fetal catabolism and present in relatively high concentration in the allantoic fluid, diffuse more rapidly into maternal blood than do larger ions whose transfer is linked to maintain electroneutrality. To test this site and mechanism we propose to do experiments in unanesthetized fetal sheep that will: 1) differentiate between the chorioallantoic membrane and fetal body as the origin of the potential; 2) characterize the dependence of the voltage on pH, temperature, and umbilical blood flow; and 3) test its vulnerability of blockers of specific ion pumps. The experiments will use a multifilament dialysis coil to modify the composition of the allantoic fluid, newly developed Ag-AgCl electrodes to measure the potential, and intra-uterine ventilation. These techniques were not available hitherto and they will permit critical testing of the hypotheses. This work relates to an understanding of the movement of water and electrolytes between mother and fetus and is thus relevant to understanding and treatment of diseases associated with fetal edema, oligo- and polyhydramnios, and impaired fetal growth.